Discover the world of optical illusions where your
eyes and your brain can fool you. Here you will find some of the best illusions
and brain teasers around (and related resources).

Our visual system is designed to allow us functioning in
three-dimensional environment which is filled with light, shading, color,
texture, and full of objects of different sizes at different distances.
Most of us take vision for granted without realizing how complex and difficult
task our brain is performing in order to correctly represent the "real outside
world". Our perception is created by our brain's interpretation of visual
information and sometimes it results in fascinating visual illusions. Our mind
gets "actively" involved in interpreting the perceptual input rather than
passively recording the input. It does not always accurately represent that
input.

Visual Illusions can be categorized as:

Physical Illusions: here the illusion has already
occurred before light has entered the eye [ mirage, rainbow, straw in a
glass of water ]

Examples

These are examples of physical illusions, where what we see is not
really there, but where the cause of the illusion in the behavior of
light before it reaches the eye.

Mirage Phenomena

A mirage is a natural illusion that is an optical phenomenon. It is produced by the reflection of light when it passes into a
layer of hot air lying close to a heated ground surface (known as a
temperature inversion). In deserts, mirage may create the illusion
of a lake in the distance (which is actually an image of the sky reflected
by the hot air (temperature inversion layer).

The most commonly observed mirage is the inferior mirage,
which is usually seen as a shiny patch on a hot surface that is the
reflection of the sky. Summer sunlight may raise the temperature of an
exposed surface as much as 80°C or more than the air not far above it. A
strong temperature gradient is established just above the surface, and
with it a relatively thin layer of reduced density.

Inferior Mirage

Formation of inferior image

The air is normal except for a thin heated layer, so the rays are mostly
straight lines, but strongly curved upwards in the heated layer. An object
AB appears to the observer at O reflected at A'B', inverted and below the
horizon, in front of the shining pool that is the reflection of the sky.
Except for inversion, the object is not distorted. Note that the rays to
the top and bottom of the object are crossed. This crossing is necessary
to produce the reflection. The object AB is also visible by direct rays in
the normal way. It is not very unusual to see such inverted images, but in
many cases there is nothing to be imaged, so just the shining pool of the
sky is seen. The inferior mirage with this geometry is frequently seen
while driving in the summer, and cars ahead may be seen reflected in it.

A superior mirage occurs when the air below the line of
sight is colder than that above. This is called a temperature inversion,
since it does not represent the normal equilibrium temperature gradient of
the atmosphere. Since in this case the light rays are bent down, the image
appears above the true object, hence the name superior. They are in
general less common than inferior mirages, but when they do occur they
tend to be more stable, as cold air has no tendency to move up and warm
air no tendency to move down.

Superior mirage - photo by Carlos Santos
Carlos photographed this horizon line at 19:39 pm on
the Semptember 5th, 2007 on the beach of Furadouro in Ovar, Portugal.

Superior mirage detail - photo by Carlos Santos

Formation of superior mirage (source: Sechrist et al. 1989)

Superior images can be straight up or upside down, depending on the
distance of the true object and the temperature gradient. Often the image
appears as a distorted mixture of up and down parts.

The fata morgana is a complex mirage in which distant
objects are distorted as well as elongated vertically. For example, a
relatively flat shoreline may appear to have tall cliffs, columns, and
pedestals. The phenomenon occurs under much the same meteorological
conditions as the superior mirage with inversion, and contains features of
both towering and inversion.

Antisolar Rays

Antisolar rays are sometimes seen in the direction opposite the sun,
converging to the antisolar point (towards the shadow of your head). They
are caused by sunlight being scattered from aerosols like dust particles
and water vapor. Clouds or terrain may block the sun in some places, which
causes rays of light to become visible. The sun's rays are all parallel to each
other, because the sun is very far away. However, perspective causes the
rays seemingly to diverge from the sun's position, or, in the case of antisolar rays, converge to the point opposite the sun (antisolar point).

Flattening of the Sun and Moon

The temperature (and pressure) gradient in the atmosphere causes bending
of light rays. This is called atmospheric refraction. Light rays from the
low sun or moon will refract more closer to the horizon. This is why the
setting or rising sun (and moon) appears flattened: the light rays from
the lower part of the sun's disk refract more than the rays emerging from
the top, and the vertical angle over which you see the sun is decreased -
it is flattened.

Green Flash

Green flashes and green rays are rare optical phenomena that occur
shortly after sunset or before sunrise, when a green spot is visible for a
short period of time above the sun, or a green ray shoots up from the
sunset point. It is usually observed from a low altitude where there is an
unobstructed view of the horizon, such as on the ocean.

Its explanation lies in refraction of light (as in a prism) in the
atmosphere and is enhanced by atmospheric layering. Whilst we would expect
to see a blue light, the blue is dispersed (this is why the sky is blue)
and only the green light remains visible.

With slight magnification, a green rim on the top limb of the solar
disk can be seen on most clear-day sunsets. However the flash or ray
effects require a stronger layering of the atmosphere and a mirage which
serves to magnify the green for a fraction of a second to a couple of
seconds.

When light passes from one substance to another, it changes direction.
This effect is familiar to all of us from observations in bath tubs and
swimming pools, and it is called refraction. The effect is usually dealt
with as if light were rays that literally bend in direction as they pass
through the boundary, but in fact the elegant explanation of what happens
relies on the wave theory of light.